This invention relates to composite ceramic powders and a method of making the same wherein zirconium oxide exhibits only the tetragonal crystalline phase in aluminum oxide by finely dispersing zirconium oxide crystallites within fine particles of aluminum oxide. This invention provides ceramic powder raw material capable of obtaining zirconia dispersed ceramics having excellent mechanical strength wherein zirconium oxide particles are uniformly and finely dispersed in an aluminum oxide matrix, when such composite ceramic powders are used as a raw material.
Ceramic sintered bodies having zirconium oxide particles finely dispersed in other ceramic matrices are called Zirconia Dispersed Ceramics (abbreviated as ZDC). It is known that the addition of zirconium oxide (zirconia) significantly increases toughness. (For example, "Bulletin of the Ceramic Society of Japan" Vol. 17 (1982), No. 2, pp 106-111 and Japanese Patent Laid-Open Pub. No. 86413/1977) It is believed that the reason why toughness is generated is as follows: finely dispersed zirconium oxide particles are present in the form of a tetragonal symmetry in a ceramic matrix and this tetragonal zirconium oxide is transformed into a monoclinic symmetry when rupture cracks are propagated. In order for zirconium oxide to exhibit a tetragonal symmertry in the ceramic matrix, its size must be smaller than the critical particle diameter. For example, when zirconium oxide is present in an aluminum oxide (alumina) matrix, it is said that the critical particle diameter of zirconium oxide is the order of 5000 .ANG.ngstroms. Toughness is scarcely enhanced unless zirconium oxide having sufficiently small diameter is dispersed. Accordingly, in order to prepare zirconia dispersed ceramics having high toughness, it is necessary to consider which way the finely zirconium oxide can be dispersed in producing raw material powder.
An example of zirconia dispersed ceramics is an aluminum oxide-zirconium oxide system. One well-known method to prepare this system is such that raw material powders are prepared by mechanically pulverizing and mixing aluminum oxide and zirconium oxide. It is difficult by this method to prepare sintered bodies wherein zirconium oxide having a particle size of 5000 .ANG.ngstroms or smaller is dispersed, unless the raw material powders, particularly zirconium oxide is sufficiently small. Even when zirconium oxide is sufficiently small, the mixing may not be sufficient enough to provide a good result. When zirconium oxide is locally agglomerated due to insufficient mixing, the agglomerates grow to coarse particles by sintering, and may lead to reduction in strength in many cases. In another method known in the art, water-soluble aluminum and zirconium salts are dissolved in water; the aqueous solution is made basic by the addition of ammonia and the like to simultaneously precipitate aluminum hydroxide and zirconium hydroxide; and these precipitates are calcined to form mixed powders of aluminum oxide and zirconium oxide. In the case of this method, the calcination results in the growth of particles and the aggregation between particles becomes significant. Thus, the ceramic raw material powders obtained in this method do not meet two requirements that the powders should have (i.e., fine particle size and good dispersibility), and therefore it is difficult to obtain dense sintered bodies by this method.
A further method is known in the art which comprises the steps of using an aluminum alkoxide and a zirconium alkoxide to form a uniform sol, heating the sol to form a gel, sufficiently drying the gel, then pulverizing it, and thereafter molding and sintering. (For example, see J. Am. Ceram. Soc., Vol. 61, No. 1 (1981) pp 37-39). While this method provides good dispersion of the zirconium oxide, the operations are complex and the method is not preferred from the standpoint of economy.
In sintered bodies of aluminum oxide having zirconium oxide dispersed therein which are prepared by the prior art methods as described above, it is very difficult to prepare sintered bodies wherein zirconium oxide particles are sufficiently small in size and wherein the majority of the zirconium oxide particles are in a tetragonal symmetry. We have carried out studies with respect to the raw material powders from which molded sintered bodies having excellent mechanical strength can be relatively readily prepared such that zirconium oxide is uniformly finely dispersed in aluminum oxide and wherein the majority of zirconium oxide particles are in a tetragonal symmetry, and thus have arrived at the present invention.